1. Field of the Invention
Embodiments described herein relate generally to a magnetic resonance imaging apparatus and a magnetic resonance imaging method.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) placed in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image on the basis of MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
An MRI apparatus has a gradient magnetic field coil that applies gradient magnetic fields to an imaging region so as to provide an MR signal with spatial positional information. The gradient magnetic field coil generates significant heat during imaging because of pulse electric currents repeatedly supplied thereto.
If the temperature of the gradient magnetic field coil rises, the temperature of closely disposed iron shims rises, this changes the magnetic field in the imaging space, and thereby the center frequency of the magnetic resonance of hydrogen atoms in the object also varies.
In the following explanation, the center frequency of the magnetic resonance of hydrogen atoms is simply referred to as the resonance center frequency.
The center frequency of an RF pulse such as a fat suppression prepulse and a 90 degrees excitation pulse is selected during an imaging preparation step such as a prescan, for example, on the basis of the Larmor frequency of hydrogen atoms determined in accordance with the intensity of the static magnetic field.
However, if the resonance center frequency of the hydrogen atoms in fat tissue shifts because of the heat generation of the gradient magnetic field coil during imaging after the conditions of the fat suppression prepulse are selected in the imaging preparation step, sufficient effect of the fat suppression are not obtained in some cases.
Especially, in continuous imaging which takes a long time such as dynamic imaging, the amount of heat generation of the gradient magnetic field coil increases following time elapse from the start of imaging, and accompanied with this, the shift amount of the resonance center frequency increases sometimes. In this case, images whose data are acquired at later points in time suffer influence of more degradation of the effect of the fat suppression prepulse, and sometimes satisfactory images are not obtained.
Then, the MRI apparatus written in Japanese Patent Application Laid-open (KOKAI) Publication No. 2012-30051 preliminarily stores the relationship between the amount of the temperature variation of the gradient magnetic field coil and the resonance center frequency as shift data, and measures the temperature of the gradient magnetic field coil plural times during imaging. Then, the center frequency of the RF pulses is corrected in each of the vacant periods selected by a user, in such a manner that the center frequency of the RF pulses accords with the resonance center frequency determined by the measured temperature variation and the shift data. Thereby, degradation of the effects of the RF pulses such as the fat suppression prepulse is prevented.
Although the invention of Japanese Patent Application Laid-open (KOKAI) Publication No. 2012-30051 has excellent functions and effects, it is preferable that the center frequency of the RF pulses is as close to the actual resonance center frequency at transmission timing of the RF pulses as possible.
Therefore, a novel technology to set the center frequency of the RF pulses more appropriately than the conventional technology regardless of the shift of the resonance center frequency caused by heat generation of a gradient magnetic field coil has been desired in MRI.